Laura M. Harmon

Cleveland State University, Cleveland, Ohio

George Y. Baaklini

Glenn Research Center, Cleveland, Ohio

Ultrasonic Resonance Spectroscopy of

Composite Rims for Flywheel Rotors

NASA/TM—2002-211104

September 2002
The NASA STI Program Office . . . in Profile

Since its founding, NASA has been dedicated to

the advancement of aeronautics and space

science. The NASA Scientific and Technical

Information (STI) Program Office plays a key part

in helping NASA maintain this important role.

The NASA STI Program Office is operated by

Langley Research Center, the Lead Center for

NASA’s scientific and technical information. The

NASA STI Program Office provides access to the

NASA STI Database, the largest collection of

aeronautical and space science STI in the world.

The Program Office is also NASA’s institutional

mechanism for disseminating the results of its

research and development activities. These results

are published by NASA in the NASA STI Report

Series, which includes the following report types:

• TECHNICAL PUBLICATION. Reports of

completed research or a major significant

phase of research that present the results of

NASA programs and include extensive data

or theoretical analysis. Includes compilations

of significant scientific and technical data and

information deemed to be of continuing

reference value. NASA’s counterpart of peerreviewed formal professional papers but

has less stringent limitations on manuscript

length and extent of graphic presentations.

• TECHNICAL MEMORANDUM. Scientific

and technical findings that are preliminary or

of specialized interest, e.g., quick release

reports, working papers, and bibliographies

that contain minimal annotation. Does not

contain extensive analysis.

• CONTRACTOR REPORT. Scientific and

technical findings by NASA-sponsored

contractors and grantees.

• CONFERENCE PUBLICATION. Collected

papers from scientific and technical

conferences, symposia, seminars, or other

meetings sponsored or cosponsored by

NASA.

• SPECIAL PUBLICATION. Scientific,

technical, or historical information from

NASA programs, projects, and missions,

often concerned with subjects having

substantial public interest.

• TECHNICAL TRANSLATION. Englishlanguage translations of foreign scientific

and technical material pertinent to NASA’s

mission.

Specialized services that complement the STI

Program Office’s diverse offerings include

creating custom thesauri, building customized

databases, organizing and publishing research

results . . . even providing videos.

For more information about the NASA STI

Program Office, see the following:

• Access the NASA STI Program Home Page

at http://www.sti.nasa.gov

• E-mail your question via the Internet to

help@sti.nasa.gov

• Fax your question to the NASA Access

Help Desk at 301–621–0134

• Telephone the NASA Access Help Desk at

301–621–0390

• Write to:

 NASA Access Help Desk

 NASA Center for AeroSpace Information

 7121 Standard Drive

 Hanover, MD 21076
Laura M. Harmon

Cleveland State University, Cleveland, Ohio

George Y. Baaklini

Glenn Research Center, Cleveland, Ohio

Ultrasonic Resonance Spectroscopy of

Composite Rims for Flywheel Rotors

NASA/TM—2002-211104

September 2002

National Aeronautics and

Space Administration

Glenn Research Center

Prepared for the

28th Annual Review of Progress in Quantitative Nondestructive Evaluation (QNDE)

sponsored by the Center for Nondestructive Evaluation, Iowa State University

Brunswick, Maine, July 29–August 3, 2001
Available from

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22100

Available electronically at http://gltrs.grc.nasa.gov
NASA/TM—2002-211104 1

Ultrasonic Resonance Spectroscopy of Composite Rims

for Flywheel Rotors

Laura M. Harmon

Cleveland State University

Cleveland, OH 44115

George Y. Baaklini

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, OH 44135

Summary

Flywheel energy storage devices comprising multilayered composite rotor systems are being studied extensively

for utilization in the International Space Station. These composite material systems were investigated with a recently

developed ultrasonic resonance spectroscopy technique. The ultrasonic system employs a continuous swept-sine

waveform and performs a fast Fourier transform (FFT) on the frequency response spectrum. In addition, the system

is capable of equalizing the amount of energy at each frequency. Equalization of the frequency spectrum, along with

interpretation of the second FFT, aids in the evaluation of the fundamental frequency. The frequency responses from

multilayered material samples, with and without known defects, were analyzed to assess the capabilities and limitations of this nondestructive evaluation technique for material characterization and defect detection. Amplitude and

frequency changes were studied from ultrasonic responses of thick composite rings and a multiring composite rim.

A composite ring varying in thickness was evaluated to investigate the full thickness resonance. The frequency

response characteristics from naturally occurring voids in a composite ring were investigated. Ultrasonic responses

were compared from regions with and without machined voids in a composite ring and a multiring composite rim.

Finally, ultrasonic responses from the multiring composite rim were compared before and after proof spin testing

to 63,000 rpm.

Introduction

Flywheel energy storage devices comprising multilayered composite rotor systems generate and store energy in

a rotating mass. The rotor is the complete rotating assembly of the flywheel, primarily composed of a metallic hub

and a composite rim, as shown in figure 1. The rim, as illustrated in figure 1, may contain several concentric composite rings with varying composite materials and geometries. Thick composite rings and a multiring composite rim

such as these were investigated in this study for use in International Space Station (ISS) flywheel systems.

Ultrasonic spectroscopy is used in the nondestructive evaluation (NDE) of materials. Previous approaches to

ultrasonic spectroscopy demonstrated its effectiveness in evaluating attenuation, velocity, and degradation in composite materials and ceramics; detecting and classifying discrete flaws, cracks, and corrosion; characterizing delaminations in composites; and analyzing multiple-layered structures and adhesively bonded joints (refs. 1 to 4). A

narrow ultrasonic signal was pulsed into a specimen with a piezoelectric broadband transducer, creating a wide

bandwidth frequency response. After ultrasound traveled through a specimen, a broadband transducer received the

ultrasonic response in the time domain. A spectrum analyzer converted the ultrasonic pulse from the time domain

to the frequency domain with a fast Fourier transform (FFT). The resulting spectrum was analyzed for amplitude

and frequency changes. For more details, the reader should consult the extensive review of previous approaches to

ultrasonic spectroscopy and its applications produced by Fitting and Adler (ref. 2).
NASA/TM—2002-211104 2

Tucker (ref. 5) developed and patented a new

approach to ultrasonic spectroscopy called ultrasonic

resonance spectroscopy (URS). This system utilizes a

continuous swept-frequency waveform in a pitch-catch

or through-transmission mode. The technique requires

performing an additional FFT on the frequency spectrum to find the fundamental resonant frequencies or

spectrum resonance spacing, which result from a

localized ultrasonic standing wave existing in a plate.

This method exhibited more advances in detecting

hidden corrosion in aluminum plates (ref. 6) and

evaluating bond quality in multiple-layered structures

(ref. 7) than previously used techniques. In addition,

the method was able to evaluate thin and thick multilayered composite rings from flywheels for the ISS

(ref. 8). The system also showed potential as a

scanning technique (ref. 9).

The current study assessed the limitations and

capabilities of URS as an NDE technique for material

characterization and defect detection. Amplitude and

frequency changes in the spectrum and spectrum resonance spacing domains were evaluated from the ultrasonic responses of thick composite rings and a

multiring composite rim. A thick composite ring with varying thickness was analyzed to investigate the full thickness resonance capabilities. The responses from areas in rings with manufactured and naturally occurring voids were

compared to those from defect-free areas. A multiring composite rim with electrical discharge machining (EDM)

notches was evaluated to determine the capabilities and limitations in detecting defects in the rim. Finally, the

multiring composite rim was investigated before and after proof spin testing to 63,000 rpm to establish baseline data

for further investigation.

Background

Resonance

The fundamental resonant frequency results from a localized ultrasonic standing wave existing in a specimen

whose thickness is equal to half of the wavelength. The fundamental resonant frequency is calculated from an equation that relates frequency to the thickness of and acoustic velocity in a plate. The fundamental relation (ref. 4)

between the frequency f, wavelength λ, and acoustic velocity c is

c f = λ (1)

Rearranging these terms yields

f c = λ (2a)

At the fundamental resonant frequency f

R, there is a half wavelength in the plate thickness and

d = λ

2 (2b)

Figure 1.—Rotor showing metallic hub with rim

 consisting of eight concentric rings.

Hub
NASA/TM—2002-211104 3

where d is the plate thickness. Solving for the wavelength λ yields

λ = 2d (3)

Substituting equation (3) into equation (2) yields

f c

d R = 2 (4)

Ultrasonic System

The commercially available ultrasonic system (ref. 5) employed in the current analysis includes a digital

processing oscilloscope, amplifier, digital-to-analog converter, and necessary computer software. The software generates a continuous swept-frequency acoustic wave and captures the frequency response of a test specimen. The

frequency sweep or interval is user defined, with capabilities from the audible range to 8 MHz (James Tucker, in

July 1999 E-mail to Laura Harmon). Figure 2 depicts the URS process with the ultrasonic system. The computer

software generates a digital-input waveform, shown in figure 2(a). The digital-input waveform is converted to an

analog signal and transmitted into the test specimen with a medium-damped direct-contact transducer. The mediumdamped transducer maintains a high level of energy while providing a wide-bandwidth frequency response. Another

medium-damped transducer receives the ultrasonic response in the time domain, shown in figure 2(b). A digital

spectrum analyzer converts the ultrasonic wave from the time domain to the frequency domain via FFT, shown in

figure 2(c). The frequency spectrum contains higher order resonance peaks or harmonics, as exhibited by the

material system under investigation. Since the value of each harmonic is an integer multiple of the fundamental frequency, the fundamental frequency is represented by the spacing between resonance peaks. Consequently, the

performance of a second FFT on the spectrum produces a peak representing the spectrum resonance spacing, or the

fundamental resonant frequency, as shown in figure 2(c). A resonance peak in the spectrum resonance spacing

domain indicates an impedance mismatch.

To eliminate the effects of a nonlinear transducer response, the system has the capability of equalizing the

amount of energy distributed to each frequency. Equalization is accomplished by coupling the transducers face to

face, as illustrated in figure 3(a). An example of the resulting input spectrum after equalization is shown in

figure 3(b). To compensate for the energy lost through attenuation, as in the present study, more energy can be

devoted to the higher frequencies with the resulting energy distribution depicted in figure 3(c).

Figure 2.—Through-transmission ultrasonic spectroscopy on Lucite sample. (a) Digital input waveform in time

 domain. (b) Digital output waveform in time domain. (c) Typical output display.

(a) (b) (c)
NASA/TM—2002-211104 4

Specimens and Experimental Procedures

Specimens

Four specimens, three composite rings and a multiring composite rim, were considered in this study. The thick

composite rings Log Drop 1.4, STD 3, and R2.6 had very similar structures. Evaluation of Log Drop 1.4 investigated changes in resonant frequencies due to thinning of various sections. Sections were milled from the ring, as

shown in figure 4. Region (a) of the ring was left intact; one section was removed from (b); and increasingly thick

sections were milled from regions (c), (d), and (e). Intentionally manufactured EDM notches and drilled holes of

varying sizes and depths were placed throughout the STD 3 ring. Ultrasonic responses were compared from regions

with and without manufactured flaws to observe changes in amplitude and resonant frequency. Ring R2.6 exhibited

regions that were affected by unintentional manufacturing flaws. Figure 5 shows three locations of interest in Ring

R2.6: (a) a defect-free region; (b) a 1.0-mm (0.04-in.) void in the ring spanning 4.1 mm (0.16 in.) perpendicular to

the ultrasonic wave path; and (c) a void cluster 7.7 mm (0.3 in.) in the ring spanning 12.2 mm (0.48 in.), also

perpendicular to the ultrasonic wave path.

The multiring composite rim consisted of multiple concentric rings similar to the thick composite rings. The

rim was evaluated before and after proof spin testing to 63,000 rpm. Before spin testing, a section of the rim was

removed. EDM notches and drilled holes of different sizes and at different locations were machined into this section

of the rim, also known as the standard rim. Responses from areas with and without detectable EDM notches on the

standard rim were compared. After spin testing of the rim free of EDM notches, measurements were taken along the

circumference every 45° in the counterclockwise direction, starting at 0° with one measurement at 15°. These

responses were compared to the responses from the areas of the standard rim section without detectable EDM

notches for evaluation before and after spin testing.

Experimental Procedures

All specimens were evaluated in the pitch-catch (i.e., send-receive) mode of ultrasonic transmission with direct

contact. The transducers were coupled to the outer diameter of the rings with couplant A (propylene glycol) or

couplant D (gel). Coaxial cables connected the transducers to the transmitter and receiver. Medium-damped transducers maximized energy while keeping the bandwidth broad.

Figure 3.—Equalization process. (a) Two transducers held face to face. (b) Spectrum

 providing equal amounts of energy to each frequency. (c) Spectrum providing greater

 amounts of energy to higher frequencies.

(a) (b) (c)
NASA/TM—2002-211104 5

Results and Analysis

Ring Log Drop 1.4

Figure 6 compares the response for section (c) with a thickness of 7.72 mm (0.304 in.) to the response for section (a) with a thickness of 10.87 mm (0.428 in.). Only the full thickness resonances were present at 0.198 MHz for

region (c) and 0.144 MHz for region (a) in the spectrum resonance spacing domain. These responses are typical of

the ultrasonic responses for each region of the ring. Resonant frequencies corresponding to locations within the

thickness were not present in the spectrum resonance spacing domain, indicating that the full thickness resonated as

one system and that the system was well bonded. The graph in figure 7 illustrates spectrum resonance spacing versus

thickness, showing the resonant frequencies at each region of the ring. As expected, thinner regions produced larger

resonant frequencies.

Figure 5.—Ultrasonic C-scan of section of Ring R2.6

 showing presence of (a) unflawed region, (b) void

 1.0 mm into ring, and (c) void cluster 7.7 mm into ring.

b

c

a

Figure 4.—Ring Log Drop 1.4 with composite sections

 removed, where region (a) remained intact; (b) had

 one section removed; and (c), (d), and (e) had

 increasingly thick sections removed.

b

c

d

e

a
NASA/TM—2002-211104 6

20

16

12

8

4

0

500

400

300

200

100

0

Amplitude, V-sec

Amplitude, (V-sec)2

0.000 1.000 2.000

Frequency, MHz

Figure 6.—Ultrasonic data from two regions of Ring Log Drop 1.4. (a) Spectrum. (b) Spectrum resonance

 spacing.

Region (c): 7.72 mm (0.304 in.)

Region (a): 10.87 mm (0.428 in.)

(a)

3.000 4.000 5.000 6.000 0.000 0.600

(b) Spectrum resonance spacing, MHz

0.200 0.400 1.000 0.800

0.375

0.325

0.275

0.225

0.175

0.125

4.00 5.00 6.00 7.00 8.00 9.00 10.00 11.00 12.00

Resonance spacing, MHz

Thickness, mm

Figure 7.—Resonance at regions (a) to (e) of Ring Log

 Drop 1.4.

0.321 (e)

0.246 (d)

0.199 (c)

0.169 (b)

0.144 (a)

Ring STD 3

Figures 8, 9, and 10 compare the ultrasonic response from a defect-free region to the responses from regions

with three manufactured flaws greater than or equal to 1.7 mm (0.07 in.) in width perpendicular to the ultrasonic

wave path. For the defect-free regions of STD 3, a sharp peak appeared at 0.148 ± 0.008 MHz, representing the full

thickness of 10.74 mm (0.423 in.) resonating. In addition, there was a resonance peak at 0.214 MHz, which corresponds to the thickness of 7.64 mm (0.301 in.). Sharp peaks indicate high impedance mismatches; however, Ring

Log Drop 1.4 did not produce multiple sharp peaks in the spectrum resonance spacing. These facts suggest the

existence of a kissing disbond (i.e., close mechanical contact without bonding) in STD 3 approximately 7.64 mm

(0.301 in.) into the ring. Energy traveled through the ring to resonate the full thickness of the ring, as well as part of

the full thickness. A previous study (ref. 7) suggested that such resonances may be indicative of a bonded sample

where some energy penetrates the bond to resonate the full thickness of the sample, and some energy reflects at the

bond. Therefore, a position at 7.64 mm (0.301 in.) into STD 3 may have a weaker bond than other locations nearer

the outer diameter. Optically, degradation cannot be seen at the depth of 7.64 mm (0.301 in.) into the ring. In addition, real-time x-ray, ultrasonic C-scan, and computed tomography scans did not detect damage at this location.

Further investigation is required to determine whether URS detected a kissing disbond or a weak bond that went

undetected by other NDE methods.

The three responses from regions with defects of widths greater than or equal to 1.7 mm (0.07 in.) perpendicular

to the ultrasonic wave path indicated amplitude reductions in both the spectrum and spectrum resonance spacing

domains. The locations of the defects were not determined from the responses, as resonances corresponding to the
NASA/TM—2002-211104 7

20

16

12

8

4

0

500

400

300

200

100

0

Amplitude, V-sec

Amplitude, (V-sec)2

0.000 1.000 2.000

Frequency, MHz

Figure 8.—(a) Optical photograph of 1.7-mm- (0.07-in.-) diameter drilled hole in Ring STD 3 with ultrasonic

 data comparing (b) frequency spectrum and (c) spectrum resonance spacing from defect-free region to

 region with drilled hole.

Drilled 1.7-mm- (0.07-in.-)

diameter hole

Defect-free

(b)

(a)

3.000 4.000 5.000 6.000 0.000 0.600

(c) Spectrum resonance spacing, MHz

0.200 0.400 1.000 0.800

1.7 mm

(0.07 in.)

20

16

12

8

4

0

500

400

300

200

100

0

Amplitude, V-sec

Amplitude, (V-sec)2

0.000 1.000 2.000

Frequency, MHz

Figure 9.—(a) Optical photograph of three drilled holes in Ring STD 3 with ultrasonic data comparing (b)

 frequency spectrum and (c) spectrum resonance spacing from defect-free region to region with three

 drilled holes.

Three drilled holes

Defect-free

(b)

3.000 4.000 5.000 6.000 0.000 0.600

(c) Spectrum resonance spacing, MHz

0.200 0.400 1.000 0.800

(a)

2.4 mm

(0.10 in.)

2.4 mm

(0.10 in.)
NASA/TM—2002-211104 8

20

16

12

8

4

0

500

400

300

200

100

0

Amplitude, V-sec

Amplitude, (V-sec)2

0.000 1.000 2.000

Frequency, MHz

Figure 10.—(a) Optical photograph of 1.7-mm (0.07-in.) by 1.0-mm (0.04-in.) EDM notch in Ring STD 3 with (b)

 frequency spectrum and (c) spectrum resonance spacing comparing EDM notch to defect-free region.

1.7-mm (0.07-in.) EDM notch

Defect-free

(b)

3.000 4.000 5.000 6.000 0.000 0.600

(c) Spectrum resonance spacing, MHz

0.200 0.400 1.000 0.800

(a)

1.0 mm

(0.04 in.)

1.7 mm

(0.07 in.)

location of the flaws did not consistently appear. The only signal that may have indicated the location of the flaw is

shown in figure 9. A resonance peak increased in amplitude at 0.309 MHz, which corresponds to the location of the

porosity network of drilled holes.

Hence, URS detected discrete defects with widths greater than or equal to 1.7 mm (0.07 in.) perpendicular to the

ultrasonic wave path as a reduction of amplitude in the spectrum and spectrum resonance spacing domains. The

locations of the defects were not determined, as resonance peaks corresponding to the locations did not consistently

appear.

Ring R2.6

Figure 11 compares responses from the defect-free region and the region with the 4.1-mm- (0.16-in.-) wide void

1 mm (0.04 in.) from the outer diameter. For the defect-free region, a sharp peak was present at 0.118 ± 0.008 MHz

representing the full thickness resonance. In addition to the full thickness resonance, a sharp peak at 0.157 ±

0.008 MHz appeared, representing an approximate thickness of 9.6 mm (0.38 in.) from the outer diameter of the

ring. This resonance may be indicative of a kissing disbond at the corresponding location; further investigation of

the results are needed to corroborate its existence. Less-defined peaks at 0.248, 0.312, and 0.494 MHz, corresponding to the approximate thicknesses of 6.1 mm (0.24 in.), 5.5 mm (0.22 in.), and 4 mm (0.16 in.) from the outer diameter, emerged in the response for the defect-free region as well. Further NDE is necessary to verify the source of the

resonance peaks, as computed tomography did not locate defects at these locations (see figs. 11(c) and 12(c)).

The void in figure 11(c) produced the ultrasonic response also shown in figure 11. The fundamental resonant

frequency corresponding to the location of the void was not resolved in the spectrum resonance spacing domain

because the spectrum sweep or interval was capable of exciting the fundamental frequency only. For a resonance to

be resolved in the spectrum resonance spacing domain, more than one peak is necessary in the spectrum domain.

Due to time constraints, another wider sweep was not performed. However, there were observable changes in the
NASA/TM—2002-211104 9

100

80

60

40

20

0

5000

4000

3000

2000

1000

0

Amplitude, V-sec

Amplitude, (V-sec)2

0.000 1.000 2.000

Frequency, MHz

Figure 11.—Regions with and without void 1.0 mm into ring R2.6. (a) Spectrum. (b) Spectrum resonance spacing.

 (c) Computed tomography image showing location of void.

Void

Defect-free

(a)

3.000 4.000 0.000 0.600

(b) Spectrum resonance spacing, MHz

0.200 0.400 1.000 0.800

(c)

12.7 mm

(0.5 in.)

1.0 mm

(0.04 in.)

response. The amplitude of the full thickness resonance at 0.118 MHz decreased by approximately 40 percent,

which means that less energy returned through the full thickness. The resonant frequency at 0.147 MHz shifted outside the error of the 0.157 MHz resonance but remained at the same amplitude. In addition, there was an overall

amplitude reduction in the frequency spectrum. A resonance corresponding to the location of the void, 1.0 mm

(0.04 in.) from the outer diameter of ring R2.6, was not resolved in the spectrum resonance spacing.

Figure 12 compares responses for the defect-free region and the region with the void cluster 7.7 mm (0.3 in.)

into the ring spanning 12.2 mm (0.48 in.) perpendicular to the ultrasonic wave path. The resonance corresponding to

the location of the void cluster, 0.210 MHz, did not emerge in the response in figure 12. The amplitude of the full

thickness resonance declined by approximately 60 percent. In addition, the frequency spectrum indicated amplitude

reductions of approximately 60 percent for the frequencies beyond 1.800 MHz. Therefore, URS evaluation detected

the presence of damage only by a reduction in amplitude.

Multiring Composite Rim With Manufactured Flaws at Different Locations

Results from the standard rim with manufactured flaws indicated that URS did not detect or locate discrete

notches with a width of 1.5 mm (0.06 in.) perpendicular to the ultrasonic wave path because of scattering and

dispersion. In contrast to Ring STD 3 and Ring R2.6, changes in amplitude were not observed because the notches

in the rim were smaller than those in the rings. In addition, resonances corresponding to the locations of the defects

did not appear.
NASA/TM—2002-211104 10

100

80

60

40

20

0

5000

4000

3000

2000

1000

0

Amplitude, V-sec

Amplitude, (V-sec)2

0.000 1.000 2.000

Frequency, MHz

Figure 12.—Void cluster in Ring R2.6. (a) Spectrum. (b) Spectrum resonance spacing. (c) Computed

 tomography image.

Void cluster

Defect-free

(a)

3.000 4.000 0.000 0.600

(b) Spectrum resonance spacing, MHz

0.200 0.400 1.000 0.800

(c)

12.7 mm

(0.5 in.)

7.7 mm

(0.3 in.)

16

12

8

4

0

160

120

80

40

0

Amplitude, V-sec

Amplitude, (V-sec)2

0.000 1.000 2.000 3.000

Frequency, MHz

Figure 13.—Ultrasonic data from multiring composite rim before and after proof spin testing. (a) Spectrum.

 (b) Spectrum resonance spacing. 

After proof spin testing

Before proof spin testing

(a)

4.000 6.000 5.000 0.000 0.080 0.120

(b) Spectrum resonance spacing, MHz

0.040 0.200 0.160
NASA/TM—2002-211104 11

Multiring Composite Rim Before and After Proof Spin Testing

Figure 13 compares two URS responses before and after proof spin testing. The resonance for the full thickness

of the rim, 82.0 mm (3.23 in.), was calculated as 0.019 MHz. The full thickness resonance did not appear in the

ultrasonic response, as expected, because ultrasound previously had penetrated only 48 mm into this rim (ref. 10).

The largest resonance peak appearing in the spectrum resonance spacing domain in figure 13(b) had two parts for

both ultrasonic responses. The resonance of the highest amplitude occurred at approximately 0.142 MHz, corresponding to 11.2 mm (0.44 in.), whereas the lower amplitude resonance occurred at approximately 0.132 MHz, corresponding to 12.1 mm (0.48 in.). The thickness of the outer ring was 12.0 mm (0.47 in.). These measurements and

calculations suggested that the outer ring resonated. Additional resonances appeared at 0.033, 0.045, and 0.066

MHz, shown in figure 13, corresponding to 48.2 (1.90 in.), 36.2 (1.43 in.), and 23.7 mm (0.93 in.) from the outer

diameter of the rim. These resonances appeared in all the responses at varying amplitudes, before and after spin

testing. Their thicknesses correspond to the thickness from the outer diameter to the fourth ring at 48 mm, third ring

at 36 mm, and second ring at 24 mm, respectively.

The fact that the URS responses for the rim before and after spin testing contained the same resonance peaks

suggests that spin testing did not introduce damage into the outer four rings of the rim. Results were in agreement

with results from ultrasonic pulse-echo scans and x-ray tomography. These URS responses will be used in the future

as signatures for rims of the same structure. Any damage existing within 48 mm of the rotor may be detected using

these signatures. Future analysis of the URS data from this rim will investigate any damage that may occur due to

fatigue testing.

URS did show potential in evaluating composite rims, as the signals before and after proof spin testing contained detailed resonances for the outer four rings. The signals acquired will serve as baseline, or signature, signals

for further evaluation and analysis of the composite rim. The appearance or disappearance of resonances, along with

changes in amplitude, will be monitored to investigate the damage state of the outer rings of the rim. This type of

analysis will improve the qualification, certification, and lifing of composite rims because these rims are designed

with a fail-safe system: when an outer ring disbonds, the flywheel rotor goes out of balance, causing a system

shutdown.

Summary of Findings

URS detected the full thickness resonance in a well-bonded thick composite ring with a single resonance in the

spectrum resonance spacing domain. In addition, full thickness resonances increased in value as the thickness decreased in different sections of the composite ring.

Evaluation of the composite standard ring with intentional EDM notches and drilled holes demonstrated that

discrete voids greater than or equal to 1.70 mm in width perpendicular to the ultrasonic wave path were detected as

an amplitude reduction in the amplitude of the spectrum and spectrum resonance spacing when compared to a

defect-free region. The locations of discrete voids were not consistently determined, since resonances corresponding

to their locations within the thickness did not appear in the spectrum resonance spacing domain. Data from the thick

composite ring also contained resonances at locations with possible kissing disbonds that were not detected by other

NDE methods.

Data from naturally occurring single and clustered voids in a thick composite ring indicated that a single

4.1-mm- (0.16-in.-) wide and 0.8-mm- (0.03-in.-) thick void 1.0 mm below the surface caused a 40-percent reduction in the full thickness resonance in the spectrum resonance spacing domain and a general decrease in the

spectrum domain. Clustered voids 7.7 mm below the surface reduced the amplitudes of the full thickness resonance

in the spectrum resonance spacing domain by 60 percent. Frequencies above 1.800 MHz in the spectrum domain

declined by 60 percent. However, the locations of the single void and the void cluster were not detected. In addition,

ultrasonic data detected possible kissing disbonds that were undetected by other NDE methods.

A comparison of URS responses from a rim made of concentric composite rings with and without EDM notches

and drilled holes detected minimal differences between responses. The amplitude changes observed in the URS

responses of the rings were not observed in the responses for the rim because the notches in the rim were smaller
NASA/TM—2002-211104 12

than those detected in the rings. The URS response detected resonant frequencies corresponding to the location of

the four outer rings, which will be used to monitor the damage state of the rim during its life. The full thickness

resonance of the standard rim was not detected because sound penetration was limited to 48 mm into the rim. The

URS response from the composite rim before and after spin testing to 63,000 rpm produced resonances of similar

amplitude and frequency, indicating the successful manufacturing of a multiring composite rim. This was in agreement with results from ultrasonic pulse-echo scans and x-ray tomography NDE by other researchers.

Conclusion

Multilayered composite rotor systems were investigated with ultrasonic resonance spectroscopy (URS) to assess

capabilities and limitations for flaw detection and material characterization. Amplitude and frequency changes in the

spectrum and spectrum resonance spacing domains were evaluated to establish the foundation for characterizing the

damage state in a composite rotor and related material systems targeted for flywheel technology of the International

Space Station.

This study assessed the capabilities and limitations of URS as an NDE method for flywheel composite rotor

evaluation. The full thickness resonance was produced in a defect-free composite ring. The presence of discrete and

clustered voids with a width greater than 1.70 mm (0.067 in.) perpendicular to the ultrasonic wave path was detected

in composite rings as an amplitude reduction in the spectrum and spectrum resonance spacing. The unique detection

of kissing disbonds by URS requires further investigation, since their existence in the composite rings and rims was

not confirmed destructively or corroborated with other nondestructive techniques. Voids of a width of 1.5 mm

(0.06 in.) were not detected in the multiring composite rim. The presence of the four outer rings was detected, as

resonances corresponding to their locations appeared. The responses before and after spin testing contained the same

resonances for the four outer rings, suggesting that damage was not introduced to the rim by proof spin testing. As a

result, the signals acquired from the multiring composite rim will be baseline signatures for comparison with signals

after fatigue testing. Based on these findings, URS is a viable tool for flight certification of flywheel rotors to be

used in the International Space Station.

References

1. Birks, Albert S.; Green, Robert E., Jr.; and McIntire, Paul: Ultrasonic Testing. American Society for Nondestructive Testing, Columbus, OH, 1991.

2. Fitting, Dale W.; and Adler, Laszlo: Ultrasonic Spectral Analysis for Nondestructive Evaluation. Plenum

Press, New York, NY, 1981.

3. Gericke, O.R.: Ultrasonic Spectroscopy. Research Techniques in Nondestructive Testing, ch. 2, R.S. Sharpe,

ed., Academic Press, London, England, 1970.

4. Krautkramer, Josef; and Krautkramer, Herbert: Ultrasonic Testing of Materials. 2nd ed., Springer-Verlag,

New York, NY, 1977.

5. Tucker, James R.: Apparatus and Method for Ultrasonic Spectroscopy Testing of Materials. U.S. Patent

5,591,913, January 7, 1997.

6. Tucker, James R.: Ultrasonic Spectroscopy for Corrosion Detection and Multiple Layer Bond Inspection.

Proceedings of the First Joint DoD/FAA/NASA Conference on Aging Aircraft, vol. II, 1998, pp. 1537–1550.

7. Chambers, J.K.; and Tucker, J.R.: Bondline Analysis Using Swept-Frequency Ultrasonic Spectroscopy.

Insight, vol. 41, no. 3, 1999, pp. 151–155.

8. Harmon, Laura M.; and Baaklini, George Y.: Ultrasonic Resonance Spectroscopy of Composite Rings for

Flywheel Rotors. Nondestructive Evaluation of Materials and Composites V: Proceedings of SPIE vol. 4336,

2001.

9. Martin, Richard; and Baaklini, George Y.: Scanning Ultrasonic Spectroscopy for Composite Flywheels.

Nondestructive Evaluation of Materials and Composites V: Proceedings of SPIE, vol. 4336, 2001.

10. Baaklini, George Y., et al.: NDE Methodologies for Composite Flywheels Certification. NASA/TM—2000-

210473, 2000. http://gltrs.grc.nasa.gov/GLTRS/ Accessed September 9, 2001.

This publication is available from the NASA Center for AeroSpace Information, 301–621–0390.

REPORT DOCUMENTATION PAGE

2. REPORT DATE

19. SECURITY CLASSIFICATION

 OF ABSTRACT

18. SECURITY CLASSIFICATION

 OF THIS PAGE

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources,

gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this

collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson

Davis Highway, Suite 1204, Arlington, VA 22202-4302, and to the Office of Management and Budget, Paperwork Reduction Project (0704-0188), Washington, DC 20503.

NSN 7540-01-280-5500 Standard Form 298 (Rev. 2-89)

Prescribed by ANSI Std. Z39-18

298-102

Form Approved

OMB No. 0704-0188

12b. DISTRIBUTION CODE

8. PERFORMING ORGANIZATION

 REPORT NUMBER

5. FUNDING NUMBERS

3. REPORT TYPE AND DATES COVERED

4. TITLE AND SUBTITLE

6. AUTHOR(S)

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

11. SUPPLEMENTARY NOTES

12a. DISTRIBUTION/AVAILABILITY STATEMENT

13. ABSTRACT (Maximum 200 words)

14. SUBJECT TERMS

17. SECURITY CLASSIFICATION

 OF REPORT

16. PRICE CODE

15. NUMBER OF PAGES

20. LIMITATION OF ABSTRACT

Unclassified Unclassified

Technical Memorandum

Unclassified

National Aeronautics and Space Administration

John H. Glenn Research Center at Lewis Field

Cleveland, Ohio 44135–3191

1. AGENCY USE ONLY (Leave blank)

10. SPONSORING/MONITORING

 AGENCY REPORT NUMBER

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546–0001

Available electronically at http://gltrs.grc.nasa.gov

September 2002

NASA TM—2002-211104

E–12949

WU–755–1A–09–00

18

Ultrasonic Resonance Spectroscopy of Composite Rims for Flywheel Rotors

Laura M. Harmon and George Y. Baaklini

Ultrasonic spectroscopy; Spectrum analysis; Resonance; Resonant frequencies;

Composite materials; Flywheels; Rotors; Nondestructive tests

Unclassified -Unlimited

Subject Categories: 38 and 20 Distribution: Nonstandard

Flywheel energy storage devices comprising multilayered composite rotor systems are being studied extensively for

utilization in the International Space Station. These composite material systems were investigated with a recently developed ultrasonic resonance spectroscopy technique. The ultrasonic system employs a continuous swept-sine waveform and

performs a fast Fourier transform (FFT) on the frequency response spectrum. In addition, the system is capable of equalizing the amount of energy at each frequency. Equalization of the frequency spectrum, along with interpretation of the

second FFT, aids in the evaluation of the fundamental frequency. The frequency responses from multilayered material

samples, with and without known defects, were analyzed to assess the capabilities and limitations of this nondestructive

evaluation technique for material characterization and defect detection. Amplitude and frequency changes were studied

from ultrasonic responses of thick composite rings and a multiring composite rim. A composite ring varying in thickness

was evaluated to investigate the full thickness resonance. The frequency response characteristics from naturally occurring

voids in a composite ring were investigated. Ultrasonic responses were compared from regions with and without machined

voids in a composite ring and a multiring composite rim. Finally, ultrasonic responses from the multiring composite rim

were compared before and after proof spin testing to 63,000 rpm.

Prepared for the 28th Annual Review of Progress in Quantitative Nondestructive Evaluation (QNDE) sponsored by

the Center for Nondestructive Evaluation, Iowa State University, Brunswick, Maine, July 29–August 3, 2001.

Laura M. Harmon, Cleveland State University, Cleveland, Ohio 44115, and George Y. Baaklini, NASA Glenn

Research Center. Responsible person, George Y. Baaklini, organization code 5920, 216–433–6016.